What It Is
Flavor scientists have identified thousands of flavor chemicals, and have broken down complex aromas, like those of strawberries or coffee, into the individual aromatic chemicals that make up a smell in an attempt to build aromas and flavors that copy nature.

What It Does
Creating artificial flavors is not easy, and it is hardly ever completely effective. For even though scientists can capture flavor molecules in concentrations of a few parts per billion, their analyses are not fine enough to allow them to create artificial flavors that duplicate the real thing. Apparently our flavor-sensing system can respond to chemicals that even the most sophisticated piece of scientific equipment can’t capture.

Give it a try. Take a bite of something, chew it, and hold it in your mouth as you try to pick apart the component flavors that form the whole. Basil is floral, sweet, minty, and anise-like, with a hint of eucalyptus and clove. Rosemary smells like mentholated pine, and vanilla is floral, sweet, and slightly spicy like nutmeg.

Sensory scientists have tools to guide them. Their main one, other than a perceptive nose and mouth and good powers of articulation, is a flavor wheel. Constructed similarly to color wheels used by painters, flavor wheels look like wagon wheels, with flavor characteristics arranged like spokes. When analyzing the flavor of a food, the researcher marks a point on each spoke. A mark near the perimeter of the wheel indicates strong intensity; a mark closer to the hub means that characteristic was more subtle. Connecting the dots creates a starburst design, called a flavor profile depicting the flavor sensations of that food.

The characteristics that make up a flavor wheel change with the food being tasted. For instance, a coffee flavor wheel would include spokes for floral and fruity flavors, but one for tasting fish would not. In general, the flavor characteristics on a flavor wheel are:

Green (Grassy), Fruity (Ester-like), Citrus, Minty (Camphoraceus), Floral (Sweet), Spicy (Herbaceous), Woody (Smoky), Roasted (Burnt), Caramel (Nutty), Bouillon (HVP), Meaty (Animal-like), Fatty (Rancid), Dairy (Buttery), Mushroom (Earthy), Celery (Soup-like), Sulfurous (Cabbage-like)

Because so much of flavor perception is aroma, we tend to use terms from the perfume industry to describe food flavors. We speak of “top notes,” meaning those aromas that hit the nose quickly and then dissipate; “mid-notes,” the main body of the flavor; and “bottom notes,” those that develop as you hold food in the mouth and that linger long after you swallow.

How It Works
In order to effectively use the hundreds of aromatic chemicals that interplay in building the flavor of food, flavor scientists categorize them by how they function. Esters, for example, are compounds that are responsible for most of the light sweet top-note aromas of fruits. On the other hand, fatty acids are strong and pungent, making up the mid- and bottom notes in fermented cheese and gamy meats.

The classes of flavor compounds are:
Acids — Carboxylic acids have a pungent sour smell that is evident in many cheeses. This group includes common organic acids like acetic acid (the acidic flavor of vinegar) and less well known but equally recognizable compounds like propionic acid, which has a sour rancid smell, and is the dominant odor in Emmental cheese. The pungency of fatty acids disappears when they react with alcohols and become sweet fruity esters. For example, butyric acid (which accounts for the rancid smell of butter) when combined with an alcohol becomes the fruity aroma in pineapples and strawberries (ethyl butyrate), in apples and pineapples (methyl butyrate), in apricots (pentyl butyrate), or in cherries (geranyl butyrate).

Alcohols — Alcohols can form floral, fruity, or fermented flavors depending on their molecular weight and what other molecules they react with. Alcohols with lower molecular weight are soluble in water and are volatile and flavorful. Ethyl maltol, the flavor of caramelized sugar and cooked fruit, is an example. As their molecular weight increases, alcohols become oily and more subtle. Decanol, the flavor of orange blossoms, and menthol are large alcohols. Alcohol molecules generate different flavors when they react with other molecules. For example, benzyl alcohol is the aroma of jasmine and hyacinth, but when it reacts with an aldehyde it becomes benzaldehyde, which is almond flavor.

Aldehydes — Aldehydes are a varied group of flavor compounds that are similar to both acids and alcohols and therefore react easily with both. Aldehydes can be floral, fruity, grassy, nutty, toasted, coffee-like, or chocolaty. One of the most commonly used aldehydes is vanillin, the flavor of vanilla. Some, like ethyl cinnamaldehyde in cinnamon, or methyl salicylate (oil of wintergreen), are so pungent they tend to dominate other flavors in a plant.

Esters — Esters are a combination of two molecules — an alcohol and an acid. Acids give vegetables and fruits tartness, and they are part of the fatty acid structure of vegetable oils. Alcohols are mostly by-products of cell metabolism in plants. Fruits in particular contain enzymes that cause acids and alcohols to combine to form aromatic esters. Apple flavor is a combination of seven esters. But banana contains just a few strong-smelling esters that give it a less complex but stronger aromatic profile.

Ketones — Ketones are polar molecules that are highly soluble in water and form bonds easily with other molecules. The acetyl-based ketones are quite subtle, giving jasmine and basmati rice their floral fragrance. Others become more pronounced from browning, giving popping corn or toasting tortillas their pleasant aroma. Some ketones produce strong dairy aromas, from the sweet, tangy aroma of cottage cheese and sour cream to the more pungent notes of blue cheese.

Iones — This subgroup of ketones produces fruit and berry flavors.

Lactones — Lactones are cyclic esters with their acid component derived from lactic acid, one of the carboxylic acids in milk. Lactones contribute to the flavors of cream, butter, honey, wine, and coconut. They are frequently added to margarine, shortening, and some baked goods to give them buttery flavors.

Phenols — Phenolic compounds account for many of the defining aromatic characteristics of spices and herbs. Eugenol, the flavor of clove, is in allspice, basil, bay leaf, cinnamon, clove, and galangal to varied degrees. Anethole is in anise, fennel, and star anise, and sotolon, a spicy caramel-tasting phenol, is in maple syrup, molasses, and tobacco. Capsaicin, the pungent part of chiles, is a phenol, as are the polyphenols in tannins.

Pyrazines — Pyrazines have the rich flavors of roasted nuts, chocolate, and browned meats. They bond easily with alcohols and acids and frequently are found in combination with them or with esters. In strong concentration they can taste musty, earthy, or fishy.

Sulfur compounds — Sulfur-containing compounds give alliums, cabbages, radishes, and mustard some of their pungency. When concentrated, sulfur compounds can be off-putting or can irritate membranes in the nose, eye, and mouth, but in small concentrations they provide an acid brightness. Much of the aromatics in roasted coffee beans come from mercaptans, which are sulfur compounds.

Terpenes – Terpenes are especially versatile, occurring in the volatile oils of many fruits and vegetables, most notably in herbs. They are volatile, which means they tend to play as top notes, providing an initial hit of light aroma, and then dissipate quickly. Most frequently terpenes have piney, woody, spicy, or citrus-like aromas. Some examples are caryophyllene, which is one of the spicy elements in allspice, black pepper, cinnamon, and clove; cineole, which gives a eucalyptus-like cooling effect to allspice, basil, bay leaf, cardamom, cubeb pepper, galangal, ginger, spearmint, and sage; citral, the citrus scent in coriander and lemongrass; and geraniol, the spicy floral quality in many tropical plants like galangal, lemongrass, and Szechwan pepper.

Science Wise
Chirality • In chemistry chirality refers to molecules that are not superimposable on each other — they are mirror images. In terms of aroma, chirality can make the same molecule smell different, depending on which direction it is facing in the olfactory receptor. For instance the terpene l-carvone, smells like spearmint but d-carvone (its mirror image) smells like caraway seed.

Reprinted with permission from THE SCIENCE OF GOOD FOOD (click link to purchase)
Text copyright 2008: David Joachim and Andrew Schloss
Published by Robert Rose Inc. 2008

Dr. Phil Handel is the Program Director for Drexel's Hospitality Management, Culinary Arts, and Food Science program.

"Science of Good Food" photograph by Horia Varlan via Flickr (Creative Commons); "Plate" photograph from FoodCollection/Getty Images.